Answer to Question #283876 in General Chemistry for kirby

Question

The NH3 molecule is trigonal pyramidal, while BF3 is trigonal planar. Which one of these molecules is flat? Why?

Answer 1

BF3 is flat

Boron has only 3 valence electrons, so when it bonds with F there will only be 3 electron pairs around the boron atom. Repulsion theory predicts that these three e- pairs should find themselves at the vertices of an equilateral triangle (bond angles of 120 degrees).